The ultrastructure of ommatidia with lateral eyes of Limulus polyphemus which had been subjected to long-term light or dark adaptation was studied. The microvilli of well light adapted retinular cells were shorter, wider and more loosely packed that those of well dark adapted cells. Subrhabdomeral pinocytotic vesicles, small coated vesicles, multivesicular bodies and secondary lysosomes were more frequently seen in the axial region of partially light adapted retinular cells. The eccentric cell dendrite of well adapted ommatidia protruded into the retinular cell cytoplasm and retinular cell microvilli were absent or reduced in the regions. The results suugest that microvillar membrane is destroyed and renewed and that changes in electrotonic coupling may occur during light-dark adaptation. Previously reported findings that epinephrine markedly reduced the latent period of the Limulus lateral eye retinular cell receptor potential must be attributed to the chlorobutanol, used as a preservative in the Parke Davis preparation. Chlorobutanol (1,1,1- trichloro-2-methyl-2 propanol) at a concentration of 0.5 micromolar decreases the latency to 65.4% of control; 1 micromolar reduces latency to 58% of control. Other trichloro compounds tested were without effect. The tribromo analogue of chlorobutanol was more effective than chlorobutanol. Sympatho-mimetic amines, blockers of mammalian gamma and beta receptors and monoamine oxidase inhibitors were without effect.